Membrane processes are well known in the art of separation science, and can be applied to a range of separations of species of varying molecular weights in liquid and gas phases (see for example “Membrane Technology and Applications” 2nd Edition, R. W. Baker, John Wiley and Sons Ltd, ISBN 0-470-85445-6).
Nanofiltration is a membrane process utilising membranes whose pores are generally in the range 0.5-5 nm, and which have molecular weight cut-offs in the region of 200-2000 Daltons. Molecular weight cut-off of a membrane is generally defined as the molecular weight of a molecule that would exhibit a rejection of 90% when subjected to nanofiltration by the membrane. Nanofiltration has been widely applied to filtration of aqueous fluids, but due to a lack of suitable solvent stable membranes has not been widely applied to the separation of solutes in organic solvents. This is despite the fact that organic solvent nanofiltration (OSN) has many potential applications in manufacturing industry including solvent exchange, catalyst recovery and recycling, purifications, and concentrations.
Polyimides have been used widely to form membranes used in separation processes, particularly gas separations, and also for separations of liquids. U.S. Pat. No. 5,264,166 and U.S. Pat. No. 6,180,008 describe processes for the production of integrally skinned asymmetric polyimide membranes which are claimed to be stable in solvents such as toluene, benzene, xylene, methyl ethyl ketone (MEK) and methyl iso butyl ketone (MIBK). These membranes are prepared as flat sheet membranes on a supporting substrate using a phase inversion technique, which results in an ultra-thin top layer of the asymmetric membrane characterised by pore sizes below 5 nm in diameter. After formation, the membranes are treated with a non-volatile conditioning agent dissolved in solvent. The conditioning agent maintains membrane properties for nanofiltration of low molecular weight solutes from organic solvents, and allows the membrane to be processed, stored and handled in a dry state. These asymmetric membranes are claimed to have utility for the separation of low molecular weight organic materials with a molecular weight in the range 300-400 Daltons from solvents with molecular weight of around 100 Daltons. The application of these membranes to solvent recovery from lube oil filtrates are described in U.S. Pat. Nos. 5,360,530; 5,494,566; 5,651,877, and in the open literature in “Solvent recovery from lube oil filtrates with a polyimide membrane” White L. S., Nitsch A. R., Journal of Membrane Science 179 (2000) pages 267-274. However polyimide membranes formed in this way from phase inversion are not stable in all solvents. In particular, they are not stable in solvents in which the polyimide forming the membrane is soluble. They tend to swell or even dissolve in such solvents.
In gas separation applications, polyimides can become plasticized, thereby losing their desirable separation properties. Crosslinking of polyimides has been investigated as a means to overcome this problem in gas separation applications. U.S. Pat. No. 4,717,393 presents photo-chemical methods for the cross-linking modification of particular polyimides containing benzophenone groups and hydrogen donor groups such as methyl groups. U.S. Pat. No. 4,981,497 describes a process to modify polyimide membranes as used for the separation of gases with amino compounds including mono-, di-, tri- or polyamines. U.S. Pat. No. 4,931,182 discloses a class of polyimide membranes for gas separations containing copolymerizable, surface-modifiable units containing both aromatic diamines and alkenylated diamines having a vinyl or vinylaryl group preferably positioned ortho to an amine functionality. The polyimide units can be crosslinked by treatment with an activating force such as a high energy electromagnetic irradiation or with a free radical source to impart high selectivity to the membrane without causing a large decrease in composite permeance. U.S. Pat. No. 6,660,062 discloses a method for crosslinking a dual layer hollow fibre, where one of the layers is a polyimide, by contacting the polyimide layer with a polyamine, using a process which comprises contacting the membrane with an alcoholic solution of an aliphatic-aromatic polyamine. US Patent Application Pub. No. 2004/0177753 A1 discloses a process for treating a polyimide membrane using dendrimers or hyperbranched polymers, which may consist of multifunctional amines. International Publication WO 2006/009520 A1 discloses a process for crosslinking a polyimide by exposing it to a cross linking agent comprising one or more amine groups. U.S. Pat. No. 6,932,859 discloses the covalent crosslinking of polyimides present in hollow fibre membranes using colvalent ester crosslinks. The crosslinking of polyimide gas separation membranes has also been disclosed in the open literature, for example in; Liu et al. Journal of Membrane Science 189 (2001) 231-239 “Chemical cross-linking modification of polyimide membranes for gas separation”; Tin et al. Journal of Membrane Science 225 (2003) 77-90 “Effects of cross-linking modification on gas separation performance of Matrimid membranes”; Shao et al. Journal of Membrane Science 238 (2004) 153-163 “Transport properties of cross-linked polyimide membranes induced by different generations of diaminobutane (DAB) dendrimers”; Shao et al. Journal of Membrane Science 267 (2005) 78-89 “The effects of 1,3-cyclohexanebis(methylamine) modification on gas transport and plasticization resistance of polyimide membranes” and Wind et al. Macromolecules 36 (2003) 1882-1888 “Solid-State Covalent Cross-Linking of Polymide Membranes for Carbon Dioxide Plasticization Reduction”. In all the above prior art the membranes are either hollow fibres or dense film membranes.